A method for reacting methanol and carbon monoxide in the presence of a noble metal catalyst to produce acetic acid is well known as a so-called “Monsanto's method”. At first, this method has been developed as a method by a homogeneous catalytic reaction in which methanol and carbon monoxide are reacted in a reaction liquid including a rhodium compound as a metal catalyst and methyl iodide as a reaction accelerator dissolved in an acetic acid solvent including water (for example, PTL 1). Thereafter as a modified method thereof, a method by a heterogeneous catalytic reaction in which a solid catalyst having a rhodium compound supported thereon is used has been developed (for example, PTL 2).
In the production process by the homogeneous catalytic reaction, the solubility of the metal catalyst into the solvent is low and therefore the reaction rate cannot be increased, thereby resulting in the increase in size of a reactor. In addition, in order to achieve the increases in reaction rate and acetic acid selectivity, and to prevent the dissolved catalyst from being precipitated, moisture is required to be present in the reaction liquid in a relatively high concentration. The moisture, however, has the problem of resulting in the hydrolysis of methyl iodide used as a reaction accelerator to cause the reduction in yield of acetic acid and the corrosion of an apparatus. Therefore, the production process by the heterogeneous catalytic reaction less causing such a problem has been developed.
Carbonylation of methanol by the heterogeneous catalytic reaction is usually a procedure in which methanol and carbon monoxide are reacted using acetic acid as a solvent in the presence of a solid catalyst having a rhodium compound supported thereon and methyl iodide as a reaction accelerator in a reactor under heat and pressure. A reaction product liquid discharged from the reactor is guided to a separation system including a unit for distillation or the like, the produced acetic acid is separated and recovered, and the remaining liquid after separation is returned to the reactor. The inside of the reactor herein is a two-phase system in which solid catalyst particles are included in the reaction liquid including acetic acid, methanol, methyl iodide and the like (more specifically, three-phase system further including carbon monoxide gas bubbles), namely, a heterogeneous system. Herein, the reaction liquid also includes methyl acetate, dimethyl ether, hydrogen iodide, water and the like as reaction by-products in addition to the above components. As the solid catalyst, a catalyst having a rhodium complex supported on an insoluble resin particle including a pyridine ring in a molecular structure is usually used.
The solid catalyst is in the form where a basic nitrogen atom included in the pyridine ring in a resin carrier is quaternized by an alkyl iodide and a rhodium complex ion [Rh(CO)2I2]− is adsorbed thereto in an ion-exchange manner. Such an ion-exchange equilibrium highly shifts to the adsorption side and substantially hardly causes the rhodium complex ion to be desorbed from the resin carrier even if acetate ions and iodine ions are present in the liquid phase in the reactor, but a problem is that when the production of acetic acid is continued for a long time, a rhodium component is gradually transferred into the liquid phase. If the amount of the rhodium component transferred into the liquid phase is large to such an extent that is unignorable, such a disadvantage that rhodium is precipitated in a flasher, contained in mist, or is incorporated to a purge flow from the process is caused, and rhodium is lost in the reactor to result in the deterioration in catalyst function, leading to the reduction in reaction rate. Furthermore, the loss of expensive rhodium results in not only the reduction in productivity but also a significant increase in catalyst cost, causing the economic efficiency of the process to be remarkably impaired.
The reason why rhodium is transferred into the liquid phase is that the pyridine ring is decomposed and desorbed from the resin carrier during the production of acetic acid for a long time to cause rhodium to be transferred into the liquid phase. That is, since the rhodium complex ion and a quaternized nitrogen atom of the pyridine ring are under an ion-exchange equilibrium and the quaternized nitrogen atom is high in affinity to the rhodium complex ion, the rhodium complex ion is not easily desorbed from the resin carrier even if other negative ions are present. If the pyridine ring is present in the liquid phase, however, a part of the rhodium complex ion supported on the resin carrier may be adsorbed to the quaternized nitrogen atom of the pyridine ring in the liquid phase to be left from the resin carrier.
In order to inhibit the rhodium component from being thus transferred into the liquid phase, a method for decreasing the concentration of a pyridine ring in a liquid phase is provided as disclosed in PTL 3. In the method in PTL 3 in which a solid catalyst having rhodium immobilized to a quaternized pyridine resin is used to produce acetic acid from methanol and carbon monoxide, a pyridine ring-containing nitrogen compound produced by decomposition of the resin can be adsorbed to a cation exchanger to be removed, thereby inhibiting the rhodium component of the solid catalyst from being flown out to the liquid phase.